1. Field of the Invention
The present invention relates to an optical module for use in optical communication modules or the like, more particularly to an optical module obtained by flip-chip mounting a surface-operating optical element on an electric circuit substrate having an optical waveguide formed thereon and filling underfill resin in between the optical element and the substrate, wherein the electrical, optical, and mechanical mounting reliability of the optical element is successfully improved and a method for manufacturing such an optical module.
2. Description of the Related Art
As a method for mounting a semiconductor element on an electric circuit substrate, there has conventionally been known a so-called flip-chip mounting method whereby a semiconductor element is mounted via a conductor bump on an electric circuit substrate. Generally, in the flip-chip mounting, after an electric circuit substrate and a semiconductor element are electrically connected and fixed to each other via a conductor bump, such a solder bump, underfill resin is filled in between the electric circuit substrate and the semiconductor element. By doing so, the bump-connected portion between the electric circuit substrate and the semiconductor element is sealed, and the other portions thereof are fixed to each other, so that the mechanical and electrical mounting reliability improve.
Meanwhile, the flip-chip mounting is adopted for mounting an optical element. For example, in the construction proposed in Japanese Unexamined Patent Publication JP-A 7-183570 (1995), as shown in FIG. 3 in section, a compound semiconductor chip 22 including a light-emitting element 25 and a silicon semiconductor chip 23 including a light-receiving element 26 are each mechanically and electrically connected to a silicon substrate 21 via a gold-made conductor bump 24. According to this proposal, the light emitted from the light-emitting element 25 is introduced into an optical waveguide 27, and then has its traveling direction changed by a reflecting surface 28 so as to enter the light-receiving element 26.
Also in a case where an optical element is flip-chip mounted, to attain sufficiently high mechanical and electrical mounting reliability, it is preferable that underfill resin is filled in between the optical element and the mounting substrate. Moreover, if there exists a gap between the optical element and the mounting substrate, there is the possibility of accidental intrusion of foreign material between the optical element and the mounting substrate, which is detrimental to establishment of optical connection. To avoid this, it is preferable to fill underfill resin in between the optical element and the mounting substrate.
However, in the case where underfill resin is filled in between the optical element and the mounting substrate having an optical waveguide formed thereon, not only it is necessary to achieve improvement in mechanical and electrical mounting reliability as conventionally intended, but it is also necessary to transmit light between the optical element and the optical waveguide with efficiency. Specifically, although it is needless to say that the underfill resin needs to exhibit light transmittability, in a case where, in the optical waveguide, the upper clad portion covering the core portion is made thin, if the refractive index of the underfill resin covering the upper clad portion is larger than the refractive index of the upper clad portion, specific light, which is basically guidedly transmitted through the optical waveguide while being trapped in the core portion having a refractive index larger than that of the clad portion, inconveniently leaks to the underfill resin arranged on the outer side of the clad portion. This makes it impossible to achieve proper light transmission.
Moreover, light leakage usually occurs between the optical waveguide and the optical element to one degree or another. In this connection, a so-called multichip module obtained by mounting a plurality of optical element chips on a single substrate faces a problem in that such leakage of light becomes stray light and couples with another optical element or optical waveguide to cause crosstalk.
The present invention has been made to solve the above stated problems with the conventional art, and has its object to provide an optical module obtained by flip-chip mounting an optical element face-down on a substrate having an optical waveguide formed thereon, and thereafter filling underfill resin in between the substrate on which the optical waveguide is formed and the optical element, wherein the mechanical, electrical, and optical mounting reliability of the optical element is successfully improved.
The invention provides an optical module comprising:
a substrate having an electric circuit formed thereon;
a surface light receiving optical element or surface light emitting optical element flip-chip mounted on the substrate with its light-receiving surface or light-emitting surface facing the substrate side;
an optical waveguide formed on the substrate, the optical waveguide having a core portion and a clad portion optically connected to the surface light receiving optical element or the surface light emitting optical element; and
underfill resin filled in between the substrate and the light-receiving surface or between the substrate and the light-emitting surface, the underfill resin covering an optical junction between the surface light receiving optical element or the surface light emitting optical element and the optical waveguide,
wherein the underfill resin is an electrically insulating material and has a refractive index equal to or smaller than a refractive index of the clad portion of the optical waveguide.
The invention further provides a method for manufacturing an optical module comprising the steps of:
forming an optical waveguide having a core portion and a clad portion on a substrate on which an electric circuit is formed;
flip-chip mounting a surface light receiving optical element or a surface light emitting optical element on the substrate with its light-receiving or light-emitting surface facing the substrate side; and
filling and curing an underfill resin of an electrically insulating material and having a refractive index equal to or smaller than a refractive index of the clad portion of the optical waveguide, in between the substrate and the light-receiving surface or between the substrate and the light-emitting surface.
According to the invention, the underfill resin, which is filled in between the light-receiving surface of the surface light receiving optical element (or the light-emitting surface of the surface light emitting optical element) flip-chip mounted on the substrate having an electric circuit formed thereon and the substrate, and covers the optical junction between the surface light receiving optical element (or the surface light emitting optical element) and the optical waveguide formed on the substrate, is of an electrically insulating material and has a refractive index equal to or smaller than the refractive index of the clad portion of the optical waveguide. Thus, the optical element can be flip-chip mounted with improved mechanical and electrical mounting reliability. Moreover, since there is no possibility of accidental intrusion of foreign material between the optical element and the mounting substrate, satisfactory optical connection can be maintained. Further, even in a case where, for example, the upper clad portion of the optical waveguide covering the core portion is made thin, it never occurs that light which is guidedly transmitted through the optical waveguide while being trapped in the core portion of the optical waveguide leaks to the underfill resin arranged on the outer side of the clad portion in the optical junction between the path and the optical element. This allows satisfactory light transmission between the optical element and the optical waveguide.
In the invention, it is preferable that, in the above stated construction, the underfill resin is of the type that its refractive index is increased by irradiation with light and has an optical guide structure formed by making the refractive index of a pathway irradiated with light coming from the optical waveguide side smaller than the refractive index of a periphery thereof.
Moreover, according to the invention, in a case where the underfill resin in use is of the type that its refractive index is increased by irradiation with light and is provided with an optical guide structure formed by making the refractive index of the pathway radiated with light coming from the optical waveguide side smaller than the refractive index of the periphery thereof, the light connected between the optical waveguide and the optical element is trapped within the optical guide structure and is thereby prevented from leaking outside of the optical guide structure. This makes it possible to effectively suppress occurrence of stray light which causes crosstalk in optical signals.
In the invention, it is preferable that, in the above stated construction, the underfill resin is of the type that its light transmittability is increased by irradiation with light, and has an optical guide structure formed by making the light transmittability of a pathway irradiated with light coming from the optical waveguide side smaller than the light transmittability of a periphery thereof.
Moreover, according to the invention, in a case where the underfill resin is of the type that its light transmittability is increased by irradiation with light and has an optical guide structure formed by making the light transmittability of the pathway irradiated with light coming from the optical waveguide side smaller than the light transmittability of the periphery thereof, when the light connected between the optical waveguide and the optical element leaks to the outside of the optical guide structure, the leakage light is attenuated by dint of the underfill resin in which the light transmittability of its portion outside the optical guide structure is low. This makes it possible to effectively suppress occurrence of stray light which causes crosstalk in optical signals.
In the invention, it is preferable that the underfill resin comprises an elastomer resin, polyimide resin, epoxy resin, silicone resin, urethane resin, acrylic resin, fluorine resin, polyolefine resin, and any other polymer resin.
In the invention, it is preferable that the underfill resin comprises a resin material in which at least a matrix contains a photopolymerization monomer and a photopolymerization accelerator.
In the invention, it is preferable that the underfill resin comprises a resin material containing a monomer or oligomer with an organic component attached thereto as a terminal group and a photopolymerization accelerator, which resin material is polymerized with dehydration or dealcoholization by irradiation with light.
As described heretofore, according to the invention, there are provided an optical module and a method for manufacturing the same, the optical module being obtained by flip-chip mounting an optical element face-down on a substrate having an optical waveguide formed thereon, and thereafter filling underfill resin in between the substrate on which the optical waveguide is formed and the optical element, wherein the mechanical, electrical, and optical mounting reliability of the optical element improves.